Some RSS feeds load with lots of interesting documents faster than every minute, for example https://reddit.com/r/all/comments/.rss
There doesn't seem to be any reason that the minutes-interval at which the Feedparser updates be restricted to integers.
I think only a change in the interval calculation and form validator is enough to add this feature, I made a fork of the node-red-nodes repository with the differences I think are needed here: https://github.com/node-red/node-red-nodes/compare/master...robertsimione:rssfeedparser-float-intervals
I do not know how to install the repo manually locally in order to test if this makes the necessary changes.